Unexpectedly autumn leads to deficit (9)
I believe the answer is:
shortfall
'deficit' is the definition.
('shortfall' can be a synonym of 'deficit')
'unexpectedly autumn' is the wordplay.
'unexpectedly' becomes 'short' (as in 'to be caught short').
'autumn' becomes 'fall' ('fall' can be a synonym of 'autumn').
'short'+'fall'='SHORTFALL'
'leads to' is the link.
(Other definitions for shortfall that I've seen before include "what we're lacking" , "Not enough money" , "Loss" , "Deficit of something required" , "Not quite what's expected" .)